Separation of close boiling compounds usually requires a process more sophisticated than conventional distillation. Extractive distillation is one of the techniques developed for this purpose. It has been applied in industrial processes, and is becoming a more and more important separation method in petrochemical industries. The main characteristic of extractive distillation is that one new solvent with high boiling-point, i.e. extractive agent, is added to the components to be separated, so as to increase the relative volatility of the targeted components. Relative volatility is a measure of the differences between the vapor pressure of the more volatile component and the vapor pressure of the less volatile component in a liquid mixture. It indicates degree of separability of two components in the mixture. Besides altering the relative volatility, the extractive agent should also be easily separated from the distillation products, that is, high boiling point difference between the extractive agent and the components to be separated is desirable. The extractive agent plays an important role in the design of extractive distillation. Therefore, selection of suitable extractive agent is essential to ensure an effective and economical design.
Ethylbenzene is a hydrocarbon compound with high commercial utilization and value. It is majorly used to produce styrene which is an intermediate for polystyrene production. Ethylbenzene may be obtained from alkylation reaction between benzene and ethylene. An alternative way for producing ethylbenzene is to recover it from a hydrocarbon mixture containing ethylbenzene which is generally produced as a byproduct stream from several petrochemical processes. The hydrocarbon mixture containing ethylbenzene usually also contain one or more hydrocarbon compound with boiling point close to boiling point of ethylbenzene, especially C8 aromatic isomers.
Attempts have been made to separate ethylbenzene from a hydrocarbon mixture. A GB patent number 1,198,592 describes a process for separating C8 aromatic isomers using a single polyfunctional distillation column. The distillation is carried out in a multiplate column having at least 250 and preferably 365 trays and a reflux ratio from 100 to 250:1 in order to achieve high purity ethylbenzene product. A large distillation column is known to have high cost of construction and high reflux rate leads to high energy consumption during operation.
A U.S. Pat. No. 3,105,017 describes a method for separating a C8 aromatic hydrocarbon mixture by distilling said mixture in the presence of a compound containing a single benzene ring substituted on the ring in at least two positions with a chloro group under conditions to separate a fraction enriched with ethylbenzene. However, this method still does not provide very high separation efficiency.
A U.S. Pat. No. 4,299,668 describes a method for separating ethylbenzene from para-xylene and/or meta-xylene in a rectification column in the presence of an extractive agent comprising pentachlorophenol as a main component mixed with one or more other compounds. Pentachlorophenol appears as white crystalline solid at room temperature with high melting point, therefore requires an additional step and energy for dissolving of pentachlorophenol in a suitable solvent before using as an extractive agent. Moreover, pentachlorophenol is extremely toxic to humans from acute ingestion and inhalation exposure.